How to add a socket for mapping node?
Unfortunately this isn't possible without modifying blender's source code. However, in the case of the mapping node, everything it does can be done with regular math nodes.

I've made some nodegroups which re-implement the mapping node accordingly:





Append or link (F1) the "Scale" "Translate" and "Rotate" nodegroups into your .blend, or put them in your startup .blend to keep them at your fingertips at all times ;)